This invention relates generally to disk caching for processor-based systems and more particularly to preserving coherency in a write-back disk cache.
Peripheral devices such as disk drives used in processor-based systems may be slower than other circuitry in those systems. There have been many attempts to increase the performance of disk drives. However, because disk drives are electromechanical in nature, there may be a finite limit beyond which performance cannot be increased.
One way to reduce the information bottleneck at the peripheral device, such as disk drives, is to use a cache. A cache is a memory device that logically resides between a device, such as a disk drive, and the remainder of the processor-based system. A cache is a memory location that serves as a temporary storage area for a device, such as the disk drive. Frequently accessed data resides in the cache after an initial access. Subsequent accesses to the same data may be made to the cache instead of to the disk drive.
Generally, two types of disk cache are used, write-through cache and write-back cache. Write-through disk cache means that the information is written both to the cache and to the corresponding disk drive. Write-back disk cache means that information is only written to the cache, and the information is only written to the corresponding disk drive when the data in the cache is being replaced with some other disk drive data. Write-back is faster than write-through cache (since writing to the slower disk is avoided on write operations) but may cause coherency problems since the data in the cache may be different (dirty) than in the corresponding disk drive. A cache line of data is dirty if the data in the cache line has been updated by the system but the corresponding disk drive has not been updated. A clean cache line is a line of data in the cache whose corresponding disk drive has the same data.
Caches are typically much smaller capacity compared to disk drives, but the most important data is kept in the cache for fastest access.
A processor-based system may use a write-back disk cache on a disk drive that is used during the normal operation of the computer and used to start (boot-up) the system. During the system start-up, the disk may be accessed by a basic input/output system (BIOS) disk request. Later in the start-up and after an operating system loads a disk drive software driver (operating system disk driver), the disk may be accessed by the operating system disk driver write request. However, the BIOS write request and the operating system disk driver write request may access or manipulate the cache and the disk drive inconsistently. Additionally, if a system crash occurs during an operation that precludes flushing the state of any dirty cache lines to the disk, the BIOS disk request may not access coherent data before the operating system disk driver loads.
Thus, there may be a need for alternative ways of implementing a disk cache.